Nanosecond-pulsed electric fields (nsPEF) applied to tissue have been shown to impart low energy in the tissue leading to very little heat production. The ability of nsPEF to penetrate into the cell to permeabilize intracellular organelles is known. (See Schoenbach et al., 2001, Bioelectromagnetics 22, 440-448; Buescher and Schoenbach, 2003, IEEE Transactions on Dielectrics and Electrical Insulation 10, 788-794; White J A, et al., 2004, J Biol Chem. 2004 May 28; 279(22):22964-72. Epub 2004 Mar. 16.).
The electric fields of nsPEFs differ from those commonly used for classical electroporation in multiple ways. First, the durations range from 1 microsecond to 200 picoseconds with rise times from 50 nanoseconds to 50 picoseconds. Second, nsPEFs can have a 20-fold larger amplitude, such as about 1 kV/cm to megavolts/cm or more. These differences in pulse parameters are believed to allow nanosecond-width pulses to penetrate into cells and electroporate organelle membranes in addition to the plasma membrane. Two separate factors are believed to produce the intracellular penetration. First, the rise time of the nsPEFs is faster than the charging time of the plasma membrane, resulting in penetration of the electric field into the cell interior. This internal field generates a current that charges the outer plasma membrane. Most cells exhibit a charging time constant of about 100 nanoseconds. This is for cells in a suspension, and would be longer for tissues. After this charging time, the resulting charge redistribution will screen out the electric field from the cell interior unless the field strength within the plasma membrane has become large enough to generate pores that provide the second mechanism for intracellular penetration.
Second, if the potential difference across the membrane exceeds about 1.6 volts, the formation of nanopores occurs within nanoseconds or less. This allows conduction current to enter the cell during the time that the pores are open. For the large field strengths used for nsPEFs, all of the molecules and organelles inside the cell will be exposed to the imposed electric field for up to hundreds of nanoseconds during each pulse due to the timing of the charging current and the open time of the field-induced pores. By applying multiple pulses, the total time of field exposure can be increased in proportion to the number of pulses applied.